Accurately measured arterial blood pressure (ABP) is one of the most critical vital signs available for both the diagnosis and the treatment of patients. An elevated ABP, for example, left untreated, may lead to a variety of cardiovascular diseases, and, as shown by Amery et al., Mortality and Morbidity Results from the European Working Party on High Blood Pressure in the Elderly Trial (EWPHE), Lancet, pp. 1349-54 (1985), to a significant decrease in life expectancy. Conversely, overestimation of ABP can lead to exposure to potential side effects from medications as well as unnecessary changes in both a patient's diet and lifestyle.
It has been recognized that ambulatory blood pressure monitoring by means of wearable sensors has the potential to enable new levels of health-related vigilance and medical care in a number of novel settings. Benefits may be realized in applications ranging from the improved diagnosis and treatment of a number of major diseases or even cardiovascular catastrophes which could occur for a wide range of patients.
Wearable blood pressure monitoring (WBPM) solutions, in various stages of technologic maturity, exist for gathering basic cardiovascular information. The Portapres® is a finger-based modality which employs the volume-clamp technique, originally developed by Penaz, to provide a continuous ABP waveform. Although it is slightly less-obtrusive than more traditional cuff-based oscillometric methods, the bulky high-bandwidth actuator that it requires is impractical for long-term patient monitoring. Other means for measuring ABP, such as pulse transit time (PTT) and the second-derivative of the photoplethysmograph (PPG), are currently at the research stage of development. These methods typically require additional information from other sensors, such as an electrocardiogram (ECG), and may be prone to motion artifact issues.
Another complication that wearable blood pressure monitors must overcome is the change in blood pressure when the patient shifts the measurement point relative to the heart. The wearable blood pressure sensor disclosed by Yamakoshi et al., Long-Term Ambulatory Monitoring of Indirect Arterial Blood Pressure Using a Volume-Oscillometric Method, Med. & Biol. Eng. & Comput., 23, pp. 459-465 (1985), indirectly accounts for the change in blood pressure by monitoring pressure in a fluid filled tube stretching from the cuff to the patient's breast pocket.
Wearable biosensors (WBS), such as the ring sensors described by Rhee et al., Artifact-Resistant, Power-Efficient Design of Finger-Ring Plethysmographic Sensors, IEEE Transactions on Biomedical Engineering, vol. 48(7), pp. 795-805, (2001), and by Shaltis et al., Artifact Resistant, Power Efficient, High Speed Modulation Design for Photo Plethysmographic Ring Sensors, Annals of Biomedical Engineering, Vol. 29, Supplement 1, (S-117), (2001), both of which publications are incorporated herein by reference, permit continuous cardiovascular (CV) monitoring in a number of novel settings.
Important health benefits such as improved disease tracking and treatment are afforded from noninvasive and unobtrusive sensor assemblies capable of monitoring blood flow, pulse rate, and blood pressure, among other parameters. However, since WBS devices are to be worn without direct doctor supervision, they must be simple to use, comfortable to wear for long periods of time, and reliable under a myriad of changing environmental conditions.
A challenge unique to WBS design is the tradeoff between patient comfort, or long-term wearability, and reliable sensor attachment. Motion-based artifacts are currently one of the most significant factors limiting the acceptance of wearable biosensors. Inflatable cuffs have shown promise for both measuring arterial blood pressure and secure sensor attachments, but are known to significantly interfere with normal blood perfusion and are therefore somewhat limited in their applicability.